a. Field of the Invention
The present invention relates to a communications system for making multimedia calls.
b. Related Art
The rapidly evolving IP (Internet Protocol) data network is creating new opportunities and challenges for multimedia and voice communications service providers. Unprecedented levels of investment are being made in the data network backbone by incumbent telecommunication operators and next generation carriers and service providers. At the same time, broadband access technologies such as DSL and cable modems are bringing high speed Internet access to a wide community of users. The vision of service providers is to make use of the IP data network to deliver new voice, video and data services right to the desktop, the office and the home alongside high speed Internet access.
The importance of standards for wide spread communications is fundamental if terminals from different manufacturers are to inter-operate. In the multimedia arena, the current standard for real-time communications over packet networks (such as IP data networks) is the ITU standard. H.323. H.323 is now a relatively mature standard having support from the multimedia communications industry that includes companies such as Microsoft, Cisco and Intel. For example, it is estimated that 75% of PCs have Microsoft's NetMeeting (trade mark) program installed. NetMeeting is an H.323 compliant software application used for multimedia (voice, video and data) communication. Interoperability between equipment from different manufacturers is also now being achieved. Over 120 companies world-wide attended the last interoperability event hosted by the International Multimedia Telecommunications Consortium (IMTC), an independent organisation that exists to promote the interoperability of multimedia communications equipment. The event is a regular one that allows manufacturers to test and resolve inter-working issues.
Hitherto, there had been a number of barriers to the mass uptake of multimedia (particularly video) communications. Ease of use, quality, cost and communications bandwidth had all hampered growth in the market. Technological advances in video encoding, the ubiquity of cheap IP access and the current investment in the data network coupled with the rollout of DSL together with ISDN and Cable modem now alleviates most of these issues making multimedia and voice communications readily available.
As H.323 was being defined as a standard, it was assumed that there would be H.323-H.320 gateways that exist at the edge of network domains converting H.323 to H.320 for transport over the wide area between private networks. Therefore, implementations of H.323 over IP concentrated on communications within a single network.
However, IP continues to find favour as the wide area protocol. More and more organisations continue to base their entire data networks on IP. High speed Internet access, managed Intranets, Virtual Private Networks (VPNs) all based on IP are commonplace. The IP trend is causing H.320 as a multimedia protocol to decline. The market demand is to replace H.320 completely with H.323 over IP.
Unfortunately, unforeseen technical barriers to the real-world, wide area deployment of H.323 still exist. The technical barriers relate to the communications infrastructure at the boundaries of IP data networks.
The H.323 standard applies to multimedia communications over Packet Based Networks that have no guaranteed quality of service. It has been designed to be independent of the underlying transport network and protocols. Today the IP data network is the default and ubiquitous packet network and the majority (if not all) of implementations of H.323 are over an IP data network. Nevertheless, today, successful implementation of multimedia and voice communications are confined to Intranets or private managed IP networks because there are IP topological problems preventing the widespread deployment of H.323 between private IP networks and the public Internet or shared or managed IP networks.
The problems arise because of two IP technologies—Network Address Translation (NAT) and Firewalls.
NAT came about to solve the ‘shortage of addresses’ problem. Any endpoint or ‘host’ in an IP network has an ‘IP address’ to identify that endpoint so that data packets can be correctly sent or routed to it and packets received from it can be identified from where they originate. At the time of defining the IP address field no-one predicted the massive growth in desktop equipment. After a number of years of global IP deployment, it was soon realised that the number of endpoints wanting to communicate using the IP protocol would exceed the number of unique IP addresses possible from the address field. To increase the address field and make more addresses available would have required the entire IP infrastructure to be upgraded. (The industry is planning to do this with IP Version 6 at some point).
The solution of the day is now referred to as NAT. The first NAT solution, which is referred to as simple NAT in IETF RFC1631, uses a one-to-one mapping, came about before the World-Wide Web existed and when only a few hosts (e.g. email server, file transfer server) within an organisation needed to communicate externally to that organisation. NAT allows an enterprise to create a private IP network where each endpoint within that enterprise has an address that is unique only within the enterprise but is not globally unique. These are private IP addresses. This allows each host within an organisation to communicate (i.e. address) any other host within the organisation. For external communication, a public or globally unique IP address is needed. At the edge of that private IP network is a device that is responsible for translating a private IP address to/from a public IP address—the NAT function. The enterprise will have one or more public addresses belonging exclusively to the enterprise but in general fewer public addresses than hosts are needed either because only a few hosts need to communicate externally or because the number of simultaneous external communications is smaller. A more sophisticated embodiment of NAT has a pool of public IP addresses that are assigned dynamically on a first come first served basis for hosts needing to communicate externally. Fixed network address rules are required in the case where external equipment needs to send unsolicited packets to specific internal equipment.
Today we find that most private networks use private IP addresses from the 10.x.x.x address range. External communications are usually via a service provider that offers a service via a managed or shared IP network or via the public Internet. At the boundaries between each of the networks NAT is applied to change addresses to be unique within the IP network the packets are traversing. Simple NAT changes the complete IP address on a one-to-one mapping that may be permanent or dynamically created for the life of the communication session.
A consequence of NAT is that the private IP address of a host is not visible externally. This adds a level of security.
Web Servers, Mail Servers and Proxy Servers are examples of hosts that would need a static one-to-one NAT mapping to allow external communications to reach them.
While computers and networks connected via a common IP protocol made communications easier, the common protocol also made breaches in privacy and security much easier too. With relatively little computing skill it became possible to access private or confidential data and files and also to corrupt that business information maliciously. The industry's solution to such attacks is to deploy ‘firewalls’ at the boundaries of their private networks.
Firewalls are designed to restrict or ‘filter’ the type of IP traffic that may pass between the private and public IP networks. Firewalls can apply restrictions through rules at several levels. Restrictions may be applied at the IP address, the Port, the IP transport protocol (TCP or UDP for example) or the application. Restrictions are not symmetrical. Typically a firewall will be programmed to allow more communications from the private network (inside the firewall) to the public network (outside the firewall) than in the other direction.
With the birth of the World-Wide Web it has become increasingly difficult to apply firewall rules just to IP addresses. Any inside host (i.e. your PC) may want to connect to any outside host (the web server) dotted around the globe. The concept of a ‘well known port’ is applied to the problem. A port identifies one end of a point-to-point transport connection between 2 hosts. A ‘well-known port’ is a port that carries one ‘known’ type of traffic. IANA, the Internet Assigned Number Authority specifies the pre-assigned well-known ports and the type of traffic carried over them. For example port 80 has been assigned for web surfing (http protocol) traffic, port 25 Simple Mail Transport Protocol etc.
An example of a firewall filtering rule for Web Surfing would be:                Any inside IP address/any port number may connect to any outside IP address/Port 80 using TCP (Transport Connection protocol) and HTTP (the application protocol for Web Surfing).        
The connection is bi-directional so traffic may flow back from the Web Server on the same path. The point is that the connection has to be initiated from the inside.
An example of a firewall filtering rule for email may be: Any outside IP address/any port number may connect to IP address 192.3.4.5/port 25 using TCP and SMTP.
The NAT function may change the destination IP address 192.3.4.5 to 10.6.7.8 which is the inside address of the mail server.
Filtering rules such as the following are frowned upon by IT managers:                Any inside IP address/any port number may connect to any outside IP address/any port number for TCP or UDP and vice versa.        
Such rules are tantamount to opening up the firewall, as it is too broad a filter.
Both NAT and firewall functions prevent H.323 communication working where NAT and firewall functions exist between the endpoints. This will typically be the case when the endpoints are in different private networks, when one endpoint is in a private network and the other endpoint is in the Internet or when the endpoints are in different managed IP networks.
H.323 has been designed to be independent of the underlying network and transport protocols. Nevertheless, implementation of H.323 in an IP network is possible with the following mapping of the main concepts:
H.323 address:IP addressH.323 logical channel:TCP/UDP Port connection
In the implementation of H.323 over IP, H.323 protocol messages are sent as the payload in IP packets using either TCP or UDP transport protocols. Many of the H.323 messages contain the H.323 address of the originating endpoint or the destination endpoint or both endpoints. In the IP world this means we have IP addresses inside an H.323 message that is sent in an IP packet whose header contains the IP addresses of source and destination hosts.
However, a problem arises in that simple NAT functions will change the IP addresses of source and destination hosts without changing the H.323 addresses in the H.323 payload. This causes the H.323 protocol to break and requires intermediary intelligence to manipulate H.323 payload addresses.
Because of the complexity of multimedia communications, H.323 requires several logical channels to be opened between the endpoint. Logical channels are needed for call control, capabilities exchange, audio, video and data. In a simple point-to-point H.323 multimedia session involving just audio and video, at least six logical channels are needed. In the IP implementation of H.323, logical channels are mapped to TCP or UDP port connection, many of which are assigned dynamically.
Another problem arises in that firewall functions filter out traffic on ports that they have no rules for. Either the firewall is opened, which defeats the purpose of the firewall, or much of the H.323 traffic will not pass through.
H.323 communication is therefore an anathema to firewalls. Either a firewall must become H.323 aware or some intermediary intelligence must manipulate the port assignments in a secure manner.
One possible solution to this problem would be a complete IP H.323 upgrade. This requires:                H.323 upgrade to the simple NAT function at each IP network boundary. The NAT function must scan all H.323 payloads and consistently change IP addresses.        H.323 upgrade to the firewall function at each IP network boundary. The firewall must understand and watch all H.323 communication so that it can open up the ports that are dynamically assigned and must filter all non-H.323 traffic on those ports.        Deployment of H.323 intelligence at the boundary or in the shared IP network to resolve and arbitrate addresses. IP addresses are rarely used directly by users. In practice, IP address aliases are used. Intelligence is needed to resolve aliases to an IP address. This H.323 function is contained within H.323 entities called GateKeepers.        
The disadvantages of this possible solution are:                Each organisation/private network must have the same level of upgrade for H.323 communication to exist.        The upgrade is costly. New functionality or new equipment must be purchased, planned and deployed. IT managers must learn about H.323.        The continual parsing of H.323 packets to resolve the simple NAT and firewall function places a latency burden on the signal at each network boundary. The latency tolerance for audio and video is very small.        
As a result of these problems, the H.323 protocol is not being used for voice and multimedia communications when there is a firewall or network address translation (NAT). One approach has been to place H.323 systems on the public side of the firewall and NAT functions. This allows them to use H.323 while also allowing them to protect the remainder of their network. The disadvantages of this are:
1. The most ubiquitous device for video communications is the desktop PC. It is nonsensical to place all desktop computers on the public side!
2. The H.323 systems are not protected from attackers on the public side of the firewall.
3. The companies are not able to take advantage of the potentially ubiquitous nature of H.323, since only the special systems will be allowed to conduct H.323 communications.
4. The companies will not be able to take full advantage of the data-sharing facilities in H.323 because the firewall will prevent the H.323 systems from accessing the data. Opening the firewall to allow data-transfer functions from the H.323 system is not an option because it would allow an attacker to use the H.323 system as a relay.
H.323 is not the only protocol now being used for real-time voice and multimedia communications over IP networks. SIP (Session Initiation Protocol as define in the IETF RFC 2543), MGCP (Media Gateway Control Protocol), H.248 (sometimes referred to as Megaco—ITU's equivalent of MGCP,) have gained industry acceptance. All these protocols suffer the identical infrastructural problems caused by firewalls and NATs and cannot traverse them unaided.